The present invention relates to receivers for demodulating angle modulated broadcast signals. Specifically, a circuit for processing angle modulated waves from a broadcast signal also containing amplitude modulation components is provided.
Recently there have been proposed systems for generating stereophonic broadcast signals in the low frequency radio spectrum currently authorized for amplitude modulation broadcasts. In one system proposed by the Magnavox Consumer Electronics Company, as set forth in FCC docket 21313, In the Matter of AM Stereophonic Broadcasting; hereby incorporated by reference, two stereophonically related signals R (t) and L (t) are summed together to form a summation signal. The summation signal modulates the amplitude of the broadcast signal comprising a monophonic broadcast received by listeners having radio receivers of conventional design. The stereophonic related signals are also subtractively combined to provide a difference signal L (t)-R (t). The difference signal is used to linearly phase modulate the broadcast carrier. Receivers of conventional design are transparent to the linear phase modulation, and are not effected by the presence of the difference signal. However, receivers which have a linear phase demodulator for demodulating the phase excursions of the intermediate frequency signal produced in a broadcast radio receiver will derive the L-R information which when combined with the AM detected L+R information will generate stereophonically related signals which may be amplified and applied to separate speakers.
Some difficulty in demodulating a stereophonic signal in accordance with the above has been found in that large negative amplitude modulation peaks, those exceeding 95%, result in a temporary loss of signal to the phase demodulator. During the time that no input signal is supplied to the phase demodulator, there can be a generation of noise in that the phase demodulator attempts to detect the phase content of noise which appears at the input during periods when a loss of intermediate frequency signals is experienced. The noise bursts which occur under these high amplitude modulation conditions can be objectionable to a listener and in general will impair the quality of stereophonic reception available in the low frequency broadcast band.